The invention relates to a method of manufacturing a glass substrate coated with a metal-oxide layer, which is used, in particular, in flat electronic display devices.
Metal-oxide layers, in particular magnesium-oxide layers, are used, inter alia, as a secondary electron emitting layer in electron multipliers. Such a layer should have a secondary electron-emission coefficient .delta..sub.max of at least 1. Dependent upon the manufacturing process, magnesium oxide has a .delta..sub.max value in the range between 3 and 25. Magnesium-oxide layers are also used on glass selection plates and the channel plate of a flat electron display as described in U.S. Pat. No. 5,497,046, which corresponds to EP-A-0,464,937 (the so-called Zeus display). The magnesium-oxide layer is necessary to generate secondary electrons and to protect the glass plates against electron bombardment. Magnesium oxide is also used as an electrode-protection material in plasma displays (PDPs) and plasma addressed LCDs (PALCs).
Magnesium-oxide layers can be manufactured, inter alia, by RF sputtering, e-beam evaporation and Chemical Vapor Deposition (CVD). Vapor-deposited magnesium-oxide layers have a 67 value of 7-12. However, these methods are expensive and not very suitable for large-scale production of glass plates having large dimensions, such as for flat electron displays. In another known method, use is made of a paste comprising MgO particles and an organic binder. The layer is obtained by means of screen printing and calcining in air at approximately 500.degree. C. The layer obtained, however, has a relatively low secondary electron-emission coefficient (.delta..sub.max &lt;5) and is porous.
A wet-chemical method of applying magnesium-oxide layers is based on the thermal decomposition of magnesium acetylacetonate in an oxygen-containing atmosphere. However, it has been found that a magnesium-oxide layer thus manufactured adheres insufficiently to glass.
Another wet-chemical method of applying (magnesium) oxide layers is the sol-gel method, as described in U.S. Pat. No. 5,412,279 and U.S. Pat. No. 5,470,606 which correspond to EP-A-0,533,256 (PHN 13.841), in the name of the current Applicant. The sol-gel method is based on the conversion of a metal-alkoxy compound in an aqueous solvent by hydrolysis and polycondensation. In this manner, for example, a silicon-dioxide layer is manufactured, using an aqueous solution of tetraethyl orthosilicate (tetraethoxy silane) as the starting material.
A drawback of magnesium-dialkoxide compounds, such as magnesium diethoxide, is, however, that the hydrolysis reaction leads to the precipitation of Mg(OH).sub.2. In addition, magnesium-dialkoxide compounds dissolve poorly in water and alcohols.
In another wet-chemical method of preparing MgO layers, magnesium ethoxide is used as the starting material. A modification with acetate and acetylacetonate groups would enable a stable coating solution to be prepared, which solution could be used to deposit MgO layers by means of a dip-coating process. By means of a subsequent curing process with UV light, it would be possible to obtain the final MgO layer.
Apart from the UV curing step, the above-described system has several other drawbacks. For example, to apply MgO layers by means of dip coating, the solutions must be stable for a long period of time. It has been found that the solutions used do not meet this requirement.